I. Field of the Invention
This invention relates to a communication circuit with malfunction self-monitoring, especially in connection with such a circuit utilized in monitoring movement of a machine member wherein the communication lines may be moving and subject to stress, fatigue and the like.
II. Description of the Prior art
Communication circuits are employed in many situations. By way of example, such a circuit may be utilized in a numerical control (NC) machine to permit communication between a drive mechanism and a remote control unit. As is well known, positioning of movable members of a machine tool may be accomplished by communicating command signals from a computer control unit or the like to a drive mechanism which controls the motion. As is similarly well known, the control unit receives a plurality of feedback signals from encoders coupled to the drive mechanism, which feedback signals indicate the actual placement and positioning of the member.
Typical of microprocessor systems in use today is that the command and feedback signals are digital, i.e., they will alternate between preassigned, opposite binary states. Because the machine and the control unit may be spaced very far apart, e.g., one hundred feet or more, communication circuits have been employed to communicate the signals therebetween. For example, to couple a feedback signal to the control unit, a typical communication circuit will include a pair of electrical lines or wires (usually twisted together for noise reduction reasons into a twisted pair) which are driven by a differential driver, the input of which is a digital feedback signal. As is well known, the differential driver will therefore drive the pair of lines such that they are not in the same state at the same time, one line driven with a signal in a state correlated to the state of the input signal, the other line driven with a signal in a different or opposite state. The twisted pair of lines may be several tens of feet long and will typically be coupled to a differential receiver at the control unit which properly amplifies and decodes the now weakened, opposite state, differential feedback signals to generate control signals for use by the control unit.
Thus, as mentioned, the twisted pair lines may be extremely long, perhaps more than one hundred feet long. Moreover, at least a portion of the lines may be moving with the machine member and thus are subject to malfunction brought about by stress, fatigue or the like. In particular, the lines in a twisted pair may short-circuit to each other, or the wires or a line may short-circuit to ground or supply or may open-circuit.
Without the ability to verify the integrity of the communication lines, the determination of a malfunction in transmission of signals produced by encoders is impractical because certain malfunctions can not be distinguished from correct operation.
As will be appreciated, failure to detect an open- or short-circuit may result in a loss of feedback from the encoder thereby leading to loss of control of the machine tool member with potentially adverse consequences.